Conventionally, compound semiconductor substrates including a group III nitride substrate such as a GaN substrate, having a mirror polished surface on which an epitaxially grown layer is formed by epitaxy, have been utilized in various semiconductor devices such as a light emitting device, a power device, and the like. There has been a problem that, if a haze occurs on a surface mirror polished as described above, a defect such as unevenness occurs in an epitaxially grown layer formed thereon, causing deterioration in the quality of the epitaxially grown layer. Therefore, various polishing methods suppressing occurrence of a haze (i.e., reducing a haze level) as described above have conventionally been proposed (for example, as to GaAs, see Patent Document 1 (Japanese Patent Laying-Open No. 11-347920)).    Patent Document 1: Japanese Patent Laying-Open No. 11-347920